The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2002-094276 filed on Mar. 29, 2002, which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a synchronous rectifying circuit, which is a kind of switching regulators, for a flyback converter.
2. Description of the Related Art
Conventionally, in a switching regulator, a synchronous rectifying circuit is known as a rectifying circuit for attaining a high efficiency. On one hand, a self-excited type flyback converter, often referred to as the ringing choke converter (RCC), which is a kind of switching regulators is most cheap among the switching regulators of about 50 watt or less, and so employed widely in an AC adapter etc. The synchronous rectifying circuit has been tried to be applied to such a self-excited type flyback converter in various manners, but sufficient effects have not been obtained. The reason why it is difficult to apply the synchronous rectifying circuit to the self-excited type flyback converter is that it is difficult to detect the turn-off timing of a rectifying diode in addition to that the switching frequency changes largely depending on amount of a load to be coupled.
In particular, in the ZVS (zero volt switching) utilizing the resonance with the leakage inductance of a transformer which has been performed recently in the switching of the primary side, the slope of reduction of a voltage from the time point near the turning-off of a rectifying diode is gentle, so that it has been increasingly difficult to apply the synchronous rectifying circuit.
Also, when a synchronous rectifying transistor is turned on at the time of turning the rectifying diode off, a large reverse pulse current flows. Thus, not only efficiency is degraded but also the synchronous rectifying transistor is broken at the worst.
In the meantime, various kinds of synchronous rectifying circuits each using a current transformer has been proposed. Examples are disclosed in U.S. Pat. No. 3,066,727 and JP-A-2002-10639.
The synchronous rectifying circuit of the current transformer type is configured to control the switching operation of a synchronous rectifying transistor by a voltage generated at a secondary winding in accordance with an output current flowing through the primary winding of the current transformer.
However, the aforesaid conventional synchronous rectifying circuit has the following drawbacks. (1) Considerable amount of a consumption of electric power by the current transformer. (2) The high price of the current transformer itself. (3) Large amount of the switching loss caused by the slow switching speed to an off state due to the gate input capacitance of the synchronous rectifying transistor, and by the moderate curvature of the increase/decrease rate of a voltage generated at the secondary winding of the current transformer. (4) Incapability of a wide range load due to the inability to switch the synchronous rectifying transistor when the load is small and the voltage level itself generated at the secondary winding of the current transformer becomes low.
The invention was made in view of such a conventional circumference and an object of the invention is to provide a synchronous rectifying circuit for a flyback converter which can eliminate the drawbacks of the current transformer type, that is, a synchronous rectifying circuit for a flyback converter which is high in efficiency, cheap and can cope with a wide range load.
In order to achieve the above object, according to a first aspect of the invention, there is provided a synchronous rectifying circuit for a flyback converter, including: a transformer having a primary winding and a secondary winding; a synchronous rectifying element coupled to the secondary winding of the transformer and performs a synchronous rectifying operation by turning on and off; an auxiliary inductance circuit coupled to the secondary winding of the transformer and has shorter energy discharge time period in comparison with the secondary winding; and a control element for turning off the synchronous rectifying element when a termination of energy discharge of the auxiliary inductance circuit is detected.
According to the synchronous rectifying circuit for a flyback converter thus configured, the circuit can be configured so as to be small in the voltage loss and also low in the cost (almost {fraction (1/10)} of the cost of the synchronous rectifying circuit using the current transformer). Further, since the auxiliary inductance circuit shorter in the energy discharge time period than that of the secondary winding of the transformer is employed and the termination of the energy discharge of the auxiliary inductance circuit is detected by the control element thereby to turn the synchronous rectifying element off, the switching loss can be made small.
According to a second aspect of the invention, in arrangement of the synchronous rectifying circuit in the aforesaid first aspect, the synchronous rectifying element includes a field effect transistor; and the control element turns off the synchronous rectifying element by discharging electric charges accumulated in a gate of the field effect transistor.
In this configuration, since the termination of the energy discharge of the auxiliary inductance circuit is detected by the control element thereby to discharge the electric charges accumulated in the gate of the field effect transistor and turn the field effect transistor off, the switching loss can be made small.
According to a third aspect of the invention, in arrangement of the synchronous rectifying circuit in the aforesaid first or second aspect, the transformer has an auxiliary winding extending from the second winding, and further including an isolation diode element coupled between the auxiliary winding and the auxiliary inductance circuit.
In this configuration, at the time of turning-on of the synchronous rectifying element by the auxiliary inductance circuit, reminder of the energy is regenerated to the auxiliary winding through the isolation diode element.
According to a fourth aspect of the invention, in arrangement of the synchronous rectifying circuit in the aforesaid third aspect, may further include a control element coupled to the auxiliary winding and turns on the synchronous rectifying element.
In this configuration, the synchronous rectifying element is turned on by the control element coupled to the auxiliary winding.
According to a fifth aspect of the invention, in arrangement of the synchronous rectifying circuit in the aforesaid first or second aspect, may further include a control element coupled to the secondary winding and turns on the synchronous rectifying element.
In this configuration, the synchronous rectifying element is turned on by the control element coupled to the secondary winding.